Field of the Invention
The present invention concerns a method and a magnetic resonance apparatus to generate multiple magnetic resonance images of an examination subject. In particular, the invention concerns techniques that allow the generation of multiple magnetic resonance images with increased resolution for respective different echo points in time using a multi-echo magnetic resonance measurement sequence.
Description of the Prior Art
Multi-echo magnetic resonance (MR) measurement sequences are known in which multiple MR images with different echo times are respectively acquired from different anatomical slices of an examined person. Due to the different echo times, the multiple MR images typically have different contrasts. The MR images with different contrasts can be applied in what are known as chemical shift techniques in which a separation of different spin species occurs.
Multi-echo MR measurement sequences are frequently implemented such that MR images are obtained at very specific and well-defined echo times. For example, the specific selection of the echo times can depend on the desired application of the MR images. One example of a typical application would be fat/water separation. The sought echo times are typically dependent on the strength of the basic magnetic field (field strength-dependent), and in fact such that the echo time (TE1) of a first MR image and the distance between the echo times of successively acquired MR images (δTE) decreases inversely proportional to the strength of the basic magnetic field of the MR system. Typical basic magnetic field strengths would be 1.5 Tesla, 3 Tesla, 5 Tesla or 7 Tesla, for example.
Various types of multi-echo MR measurement sequences are known. In a conventional multi-echo MR measurement sequence, all detected MR echoes are detected (meaning at the various echo points in time) respectively as a time period after a separated radio-frequency (RF) pulse for excitation of the transverse magnetization (RF excitation pulse). In other words: a number n of MR echoes is detected in each of separate repetition intervals (TR intervals) after an RF excitation pulse. Therefore, such techniques are also known to those skilled in the art as an n-echo n-TR approach. N-echo n-TR techniques are known in connection with the detection of gradient echoes, for example. The resolution of an MR image in the readout direction (frequency encoding direction) is typically defined by the Fourier pixel size Δx. The Fourier pixel size is a size of a field of view in the readout direction, divided by the number of readout points Nx. The field of view designates a region of an examination subject that is depicted by the MR image. The smaller the Fourier pixel size Δx, the higher the resolution. The Fourier pixel size is inversely proportional to the 0th moment of the readout gradient:Δx=2π/(γM0x).wherein γ is thereby the gyromagnetic ratio. For water protons, the gyromagnetic ratio is γ/(2π)=42.576 MHz/T. The 0th moment of the readout gradient is the time integral of the amplitude of the readout gradient during the readout time, frequently also designated as an “area” of the readout gradient. If the readout gradient is thus constant during the entire readout time, the 0th moment M0x is then the product of amplitude of the readout gradient and readout time.
In gradient echo imaging, a switch is frequently made between the excitation and readout gradients of a pre-phasing gradient pulse in the readout direction whose 0th moment has the same magnitude as the moment of the readout gradient between the beginning of the readout gradient and the echo point in time. The direction of the pre-phasing gradient pulse is typically opposite the direction of the readout gradient, such that the total moment disappears exactly at the echo point in time. The echo time is usually the time between the center of the excitation pulse and the echo point in time. For example, the echo time can also be the time between a spin echo and the echo point in time.
Since the maximum amplitude of a gradient pulse and the shortest rise time can typically be technologically and/or physiologically limited, the maximum resolution given gradient echo-based n-echo n-TR techniques is thus conventionally limited by the shortest required gradient echo time TE1, but not additionally by the shortest time difference ΔTE of successive gradient echoes. However, the total duration that is required to implement the multi-echo MR measurement sequence (measurement duration) is relatively long. Moreover, such a technique frequently extends the time interval between the detection of the different gradient echoes. This can lead to negative effects, in particular in measurements that are implemented to avoid breathing artifacts while an examined person holds his breath. Moreover, time-dependent drifts of the basic magnetic field, for example as a result of physiological processes or heating during the measurement, can lead to additional phase differences between the individual MR images with different echo points in time. A subsequent evaluation of the MR images can thereby be possible only to a limited extent, and possible quantitative analyses can be plagued with a comparably large error.
Other multi-echo MR measurement sequences are known than the n-echo n-TR-based measurement sequence described above. For example, multi-echo MR measurement sequences are also known which respectively detect multiple echoes at different echo points in time after a single RF excitation pulse. Due to the predetermined different echo points in time, given such multi-echo MR measurement sequences a maximum achievable spatial resolution is typically limited by the first echo time TE1, and additionally by the time difference between successive echoes. It is noted in particular that the time period provided for the detection of an echo is also limited in that the next echo should already be formed and detected after the time period ΔTE.
The maximum gradient strength and/or a maximum rise time and fall time of gradient fields of an MR system is frequently technologically and/or physiologically limited. For example, for the detection of gradient echoes it is frequently necessary to initially switch pre-phasing gradient pulses and to subsequently switch readout gradient fields during the readout of the gradient echo. Since the time period available for this is typically limited by the predetermined different echo points in time, the maximum 0th moment M0x of the readout gradients (and therefore the achievable spatial resolution) is frequently limited accordingly. The detection of multiple MR echoes following one RF pulse is also known to the man skilled in the art as an n-echo per TR technique.
From the preceding it is apparent that multi-echo MR measurement sequences that use an n-echo per TR technique enable a reduced measurement duration and lower movement sensitivity, but have a relatively severely limited spatial resolution of the MR images. It is also apparent that multi-echo MR measurement sequences according to the n-echo n-TR technique enable high spatial resolutions of the MR images but require a relatively long measurement duration and are movement-sensitive. Therefore, the necessity typically exists to balance between the optimization variables of spatial resolution on the one hand and measurement duration on the other hand.
In order to solve this problem, hybrid techniques are also known which combine the n-echo per TR technique with the n-echo n-TR techniques. For example, see in this regard H. Yu et al., “A Multi-echo Acquisition Method with Reduced Echo Spacing for Robust IDEAL Water-Fat Decomposition at 3T” in Proc. Intl. Soc. Mag. Reson. Med. 15 (2007) 3353. There, of the necessary six echoes three are respectively detected in a total of two successive TR intervals. The achievable spatial resolution is then still limited only by the doubled echo spacing of the n-echo per TR technique. However, the measurement duration doubles approximately in comparison to the n-echo per TR technique, and the movement sensitivity increases.